Description

Short description of error
Blender behaves weirdly when appending objects that are part of already linked group.

Exact steps for others to reproduce the error
Open Blender, Add a Cube and a Monkey, group them together and save the file as Source.blend
Make a new file called Target.blend and link the group from Source.blend
Append an object Suzanne from the Source.blend - now it is not actually appended but it is linked instead (is this also a bug?)
Select the appended Suzanne (the one that is not in the linked group) and in object properties make the object datablock local.
Now you can move the appended Suzanne but the linked one moves as well (even that the Suzanne in the Source.blend does not move)
To solve the problem you need to save the file and revert it.